Prestressed concrete



"G. C. FRANCIS 3,319,386

PRESTRESSED CONCRETE- May 16, 1967 Original Filed May 29, 1961 Jai;

v Arm/v Rp Aff I/ l zr l)/\ i e f HGB i J I :Q v :Nvt-:mon GERALD c. FRANCIS ATTO RN E YS United States Patent Oiilce p 3,319,386 Patented May 1 6, 1967 3,319,386 PRESTRESSED CONCRETE Gerald C. Francis, B0. Box 512, Lansing, Mich. 48903 Original application May 29, 1961, Ser. No. 113,457, now Patent No. 3,222,835. Divided and this application Apr. 23, 1965, Ser. No. 459,972

9 Claims. (Cl. 52-224) The present application is a division` of my copending application Ser. No. 113,457, filed May 29, 1961 now U.S. Patent No. 3,222,835.

My invention relates to prestressed concrete and more particularly to a prestressed slab form of concrete and a method of prestressing same.

Various problems have been encountered in the prestressing of concrete slabs, particularly with regard to providing substantially uniform stresses. Other problems are in the setting up and operation of the mechanisms used to provide the prestressing.

An object of the present invention is to simplify concrete prestressing by providing an improved method of setting up and operating prestressing mechanisms.

Another object of the present invention is to facilitate prestressing of a concrete slab by setting up spaced frame members connected with cables, moving one frame member relative to another to induce stress in the cables, pouring concrete between the frame members and around the cables, then releasing the frame members whereby cable stresses are imparted to the concrete.

A further object of the invention is to construct a new type of prestressed concrete slab by producing unique stresses between a central area and the periphery of the slab.

For a more complete understanding of the invention, reference may be had to the accompanying drawing illus- 12 While inducing compression stresses in the outer frame member 11.

The outer frame member is then immobilized or xed in this position by any preferred means (not shown). It will be seen that the cables 12 have assumed a position suchvthat they diverge angularly from their initial radial lines, and inwardly projected lines from the cables will circumscribe a circle, as indicated in FIG. l, concentric with the common center point or axis A of the `frame members 10 and 11.

Once iixed, a form 20 is preferably constructed as indicated in FIG. 2 on the lower side of the assembly. A concrete slab 21 is then poured between the frame members 10 and 11, enclosing the cables 12, and the connecting elements.

After the concrete has set, the form 20 is removed and the frame members 10 and 11 are released. The stresses in the cables are thereby imparted to the concrete in the well-known manner, producing a ready-made slab of uniformly prestressed concrete provided with securely retained inner and outer peripheral rims.

trating some preferred embodiments of the invention FIG. 2 is a cross-sectional view taken substantially on the line 2-2 of FIG. 1 but including the concrete as in the completed slab. v

FIG. 3 is a plan elevational view of another preferred structure as initially provided before stressing.

FIGS. l and 2 illustrate the structure and method of fabricating a circular slab, while FIG. 3 illustrates the structure and method of fabricating a polygonal slab.

The circular structure is set up by providing an inner tension frame lmember 10 and an outer compression frame member 11 spaced from the inner member 1t). A plurality of cables 12, all of equal length, connnect the members 1l) ad 11 and are secured initially with no stress on radial lines indicated by the dash lines in FIG. 1.

The frame members 10 and 11 preferably are T-shaped as shown, having respectively outwardly extending and inwardly extending feet or web flanges 10A and 11A, and peripheral head elements 10B and 11B respectively.

The cables 12 are provided at each end with connector fittings such as clevises 13 secured to equally spaced points on the flanges 10A and 11A by any means such as rivets 14 or the like.

One of the frame members, preferably the inner member 10, is secured to and immobilized by a mounting structure 15 preferably secured to a foundation structure 16.

Next, the outer frame 11 is rotated as indicated by the arcuate arrow to the lfull line position of FIG. l. This increases the distance between each cables connecting points without altering the general radial distance between the frame members 10 and 11. This induces tension stresses in the inner frame member 16 and the cables This complete integral assembly may be made in many sizes. It is contemplated for use both as an airplane hangar floor and as a table-top. It will be apparent that the number of cables used may be greater or lesser than the number shown, depending on the size and required prestre'ssing.

FIG. 3 illustrates the structure in which the present method is used to produce a noncircular slab. For convenience, a square form is shown as comprising a circular inner tension frame member 25 and a square outer compression fra-me member 26 preferably made of T section as the members in FIGS. 1 and 2 connected by cables 27 and 2S.

The outer member 26 is secured and held in shape preferably by a truss assembly made substantial enough to prevent a tendency of the outer member 26 to deform when it is rotated to the dash-line position after the inner member 25 is immobilized.

It is noted that the cables 27 and 28 are of unequal length, and thus the changes in dimension produced between their respective connecting points on rotation will vary, there being a greater change in the shorter than the longer dimensions. Thus it may be necessary to provide a greater initial degree of slack in the shorter cables 27, determined with reference to the degree of rotation required to produce the desired uniform tension in all cables.

Other than the above, the method of construction, applying tension, pouring the concrete, and releasing the frame members to transmit stresses to the concrete, will be substantially the same as the method described for the structure ofFIGS. l and 2.

It will be noted that either the inner or outer frame member could be rotated, but it is easier, due to the greater leverage, to rotate the outer.

The net result of the described methods is seen to provide a relatively simplified system for producing substantially uniform prestressing of concrete slabs which, when completed, are ready-made with integral rims and have a central open space which may have dilferent uses. When used for a hangar-door or roof, for example, the central area may be used for a center post. When used as a table top, the hole is a convenient location for a planter or the like.

Although I have described only two embodiments of the invention, it will be apparent to one skilled in the art to which the invention pertains that various changes and modifications may be made therein without departing from the spirit of the invention or the scope of the appended claims.

I claim:

1. In a prestressed concrete slab structure,

(a) an inner tension frame member having a peripheral flange extending radially outwardly therefrom,

(b) an outer compression frame member radially outwardly spaced from the inner frame member and having a peripheral fiange extending radially inwardly toward and in a com-mon plane with said inner frame member peripheral ange,

(c) a plurality of tension stressed cables connecting the inner and outer frame member flanges whereby said cables extend between said frame members with their longitudinal axes in a common plane with said flanges,

(d) means vfixing said inner frame member to prevent rotation thereof and said outer frame member being connected with said inner frame mem-ber only by said cables whereby said outer frame member can be rotated about said inner frame member to the extent permitted by said cables, and such rotation produces tension forces in said cables,

(e) a flat concrete slab disposed between said frame members and encompassing said cables, and

(f) said cables extending through said concrete slab and diverging angularly from their initial radial lines such that inwardly projected lines from said cables circumscribe a circle concentric with the common center point of said frame members.

2. The structure as defined in claim 1 and in which said inner frame member is in the form of a circle.

3. The structure as defined in claim 2 and in which said outer frame member is in the form of a circle concentric with said inner frame.

4. The structure as defined in claim 2 and in which said outer frame member is in the form of a polygon.

5. The structure as defined in claim 2 and in which said frame members are T-shaped in cross-section, the feet of the Ts forming said anges and the concrete extending between the heads of Said Ts and enclosing the feet thereof.

6. A prestressed concrete slab structure comprising (a) a central tension member,

(b) a peripheral compression member radially spaced from and co-planar with said tension member,

(c) a plurality of stressing cables secured between spaced points on said compression member and on said tension member,

(d) said cables being co-planar with each other and with said tension member and said compression member,

(e) a flat concrete slab disposed between said tension and compression members and encompassing said cables, and

(f) said cables extending through said concrete slab and diverging angularly from their initial radial lines such that inwardly projected lines from said cables circumscribe a circle concentric with the common center point of said tension member and said compression member.

7. In a prestressed concrete slab structure,

(a) an inner frame member,

(b) a peripheral frame member outwardly spaced from said inner frame member and substantially coplanar therewith,

(c) a plurality of stressing cables secured between spaced points on said inner frame member and spaced points on said peripheral frame member,

(d) means fixing one of said members to prevent rotation thereof and the other of said members being connected with said fixed member only by said cables whereby said last mentioned member can be rotated -about said fixed member to the extent permitted by said cables and such rotation produces tension forces in said stressing cables,

(e) a at concrete slab disposed Ibetween said frame members and encompassing said cables, and

(f) said cables extending through said concrete slab and diverging angularly from their initial radial lines such that inwardly projected lines from said cables circumscribe a circle concentric with the common center point of said frame members.

8. The structure as defined in claim 7 and in which said cables are all disposed with their longitudinal axes in a common plane.

9. In a prestressed concrete slab structure (a) an inner frame member,

(b) a peripheral frame member outwardly spaced from said inner frame member and substantially coplanar therewith,

(c) a plurality of stressing cables secured between spaced points on said inner frame member and spaced points on said peripheral frame member,

(d) said frame members being disposed on a common central axis, means fixing one of said members to prevent rotation thereof about said common central axis, and the other of said members being connected with said fixed member only by said cables whereby said last mentioned member can be rotated about said fixed member and about said common central axis to the extent permitted by said cables and such rotation produces tension forces in said stressing cables,

(e) a at concrete slab disposed between said frame members and encompassing said cables, and

(f) said cables extending through said concrete slab and diverging angularly from their initial radial lines such that inwardly projected lines from said cables circumscribe a circle concentric with the common central axis of said frame members.

References Cited by the Examiner UNITED STATES PATENTS 1,193,767 `8/1916 Crisell 52-82 1,390,073 9/1921 Allen 52--648 1,559,837 11/1925 Allen 52-225 1,960,328 5/1934 Breiries 52-261 2,411,651 11/1946 Darby 52-82 3,153,302 10/1964 Wheeler 52-73 3,153,303 10/1964 Wheeler 52-224 FRANK L. ABBOTT, Primary Examiner.

J. L. RIDGILL, Assistant Examiner. 

7. IN A PRESTRESSED CONCRETE SLAB STRUCTURE, (A) AN INNER FRAME MEMBER, (B) A PERIPHERAL FRAME MEMBER OUTWARDLY SPACED FROM SAID INNER FRAME MEMBER AND SUBSTANTIALLY COPLANAR THEREWITH, (C) A PLURALITY OF STRESSING CABLES SECURED BETWEEN SPACED POINTS ON SAID INNER FRAME MEMBER AND SPACED POINTS ON SAID PERIPHERAL FRAME MEMBER, (D) MEANS FIXING ONE OF SAID MEMBERS TO PREVENT ROTATION THEREOF AND THE OTHER OF SAID MEMBERS BEING CONNECTED WITH SAID FIXED MEMBER ONLY BY SAID CABLES WHEREBY SAID LAST MENTIONED MEMBER CAN BE ROTATED ABOUT SAID FIXED MEMBER TO THE EXTENT PERMITTED BY SAID CABLES AND SUCH ROTATION PRODUCES TENSION FORCES IN SAID STRESSING CABLES, (E) A FLAT CONCRETE SLAB DISPOSED BETWEEN SAID FRAME MEMBERS AND ENCOMPASSING SAID CABLES, AND (F) SAID CABLES EXTENDING THROUGH SAID CONCRETE SLAB AND DIVERGING ANGULARLY FROM THEIR INITIAL RADIAL LINES SUCH THAT INWARDLY PROJECTED LINES FROM SAID CABLES CIRCUMSCRIBE A CIRCLE CONCENTRIC WITH THE COMMON CENTER POINT OF SAID FRAME MEMBERS. 